Various systems are well-known for the detection of intruders, for instance movement sensors and both passive and active infra red detectors. It is also well-known to detect the presence of a conductive, inanimate, object by its influence on a magnetic field, for instance the position of a movable magnet relative to a reed switch. It is an object of the present invention to provide an improved method and apparatus for detecting the presence of an intruder, or the like, in a detection zone.